Drop RepackageTask.getDependencies()

Drop `RepackageTask.getDependencies` from the gradle plugin in favor of
exposing `getLibraries()` and using the callback.
Drop getDependencies
Allow URL resolution within nested JARs

Update JarURLConnection to allow the resolution of items within a nested
jar, even if the jarFile passed to the connection is several levels up.

This prevent a connection from incorrectly resolving an entry against
the wrong jar file.

See gh-1070
Add Library abstraction

Add a Library class update the LibraryCallback interface and
implementations to use it. This change is in preparation for
an addition `unpack` flag that will be required to allow the
automatic unpacking of certain nested jars.

See gh-1070
Add support for unpacking nested JARs

Update the executable JAR code to automatically unpack any entries
which include an entry comment starting `UNPACK:` to the temp folder.
The existing Maven and Gradle plugins have been updated with new
configuration options and the `spring-boot-tools` project has been
updated to write the appropriate entry comment based on a flag passed
in via the `Library` class.

This support has been added to allow libraries such a JRuby (which
assumes that `jruby-complete.jar` is always accessible as file) to work
with Spring Boot executable jars.

Fixes gh-1070
Merge branch 'gh-1070'